In general, narrowly-spaced multiple fins are arranged in an aluminum heat exchanger in order to maximize the surface area thereof, and refrigerant circulation tubes are arranged on the fins in an intricate pattern in view of improved heat exchange efficiency. In the case of a heat exchanger with such a complicated structure, when atmospheric moisture adheres on a surface of a fin or a tube (hereinafter, referred to as a “fin and the like”) as condensed water, this condensed water may stay on the surface of a fin and the like for a long time. If this occurs, an oxygen concentration cell may locally be formed to promote a corrosion reaction, resulting in the development of white rust.
Accordingly, as a technology for improving the corrosion resistance of a surface of an aluminum material, known is a method in which a chemical conversion coating film is formed by allowing the surface to contact with a chemical conversion agent. For example, as a chemical conversion agent which can be applied to a fin in an aluminum heat exchanger, proposed is a chemical conversion agent including a zirconium compound, a titanium compound, a vanadium compound and polyvinyl alcohol (see Patent Document 1). According to this technology, the chemical conversion agent can be applied on a surface of a metal substrate to supposedly improve the corrosion resistance thereof.
In recent years, the improvement in blackening resistance (hereinafter referred to as “moisture resistance”) has also been emphasized in addition to the improvement in corrosion resistance for aluminum heat exchangers. Herein, an indicator of corrosion resistance is white rust, whereas, an indicator of moisture resistance is blackening. White rust is a corrosion phenomenon generated by a corrosion factor such as oxygen, water, and chloride ions, whereas, blackening is a corrosion phenomenon generated by the existence of oxygen, water, and heat.
Meanwhile, in a case where the corrosion resistance and moisture resistance of a chemical conversion coating film formed on a surface of an aluminum heat exchanger is not sufficient, the moisture adhered on a surface of a fin and the like may promote corrosion on an aluminum surface, resulting in the development of white rust and blackening. Mineral components increases as this occurs, and they themselves generate or adsorb odors. In particular, odor generation is a serious problem for a heat exchanger used in an air conditioner. Although a hydrophilic coating film is usually formed on a chemical conversion coating film in a heat exchanger, odors of inorganic matters resulted from corrosion can not be suppressed by this hydrophilic coating film. Therefore, in order to suppress odors, it is essential to suppress the corrosion on an aluminum surface by enhancing the corrosion resistance and moisture resistance of a chemical conversion coating film formed with a chemical conversion agent.
Note that a technology of forming a hydrophilic coating film on a surface of an aluminum heat exchanger is known in which a component for forming a hydrophilic coating film comprising polyvinyl alcohol in addition to a metal from a zirconium compound, a vanadium compound and the like (see Patent Documents 2 and 3).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-231418
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2000-345362
Patent Document 3: PCT International Publication No. WO2000/022188